1. Field of the Invention:
The present invention relates generally to a power steering apparatus used in an automobile and more particularly to a servo valve device for controlling a fluid pressure motor in the power steering apparatus.
2. Description of the Prior Art:
In a heretofore known device of the type characterized herein, a nozzle-flap type servo valve is composed of a main valve member being rotatable in a valve housing, two valve chambers axially spaced in the main valve member and being sealed from each other, and first and second flap valves disposed respectively in the valve chambers and being rotatable with regard to the main valve member for controlling a stream resistance at distribution ports and injection ports, with the first and second flap valves being connected through a torsion-bar extending therebetween to a steering wheel and an output of the hydraulic motor.
Since the servo valves of the prior art have a plurality of annular recesses on the peripheral portion of the main valve member for providing communication between the conduits in the main valve to both chambers of the hydraulic motor, it is not enough to make the servo valve small in size. Since a plurality of seal elements are used, a frictional resistance caused thereby is comparatively large, thereby lowering the efficiency in rotation.
A power steering device with a servo valve, especially with a flap valve type servo valve, is generally characterized by a high response so that it turns or rotates with a high degree of sensitivity, and instantaneously, in response with a slight relative phase difference in the flap valve mechanism. However, since the sensitivity of the flap valve mechanism is sufficiently great to rapidly elevate the pressure in the fluid circuit upon an actuation thereof, vibration is created in the flap valve mechanism, thereby to unstabilize the characteristic for controlling fluid in the circuit and thereby to cause an operator to feel or sense the same as an unpleasant vibration.